You are using only on gasket and that is around the outer circumfrerence of the plate aren't you?? There is no center gasket used on the filter plat. It will limit how much the outer rim can be tightened down to seal too much. Make sure that when the outer sealing surface of the plate is flat on the block that ther is clearance between the block and the center surface where the double ended hollow bolt goes through. There has to be clearance in the center in order for it to draw cdown tight.
I make sure to have the block surface clean and dry, then put a nice coat of gun grease on the gasket to basically hold it in place and keep it somewhat flexible and it will also expande from this put it in the block and then put the plate in making sure that it is in square and tighten down. No problems so far doing it that way and have done many in the last 40 or so years.